I've come upon this problem before - somehow I don't fancy spending 3 grand on AutoCAD for something which is really a bit of fun at weekends.
So, as I'm a computer programmer, I've decided to set about writing one. It will be a basic 3-D CAD system, with some built in routines for basic aerodynamic, weight&balance, structural analysis, etc. Its not supposed to compete with the likes of CATIA, but instead to be accessible.
I'm about 35-40% of the way there at the moment, limited to working evenings and weekends so its a bit slow. Best thing though, it will be "open-source" (in effect, Free! -> I hope, as long as it doesn't end up costing too much to develop). Will keep you all posted when things get a little closer to completion. (+PM me if you're interested, or have ideas of things you might like it to do.)
cheers
dhd (alias Chris)